1. Field of the Invention
The present invention relates to a sensor having a plurality of light receiving elements and a scanning circuit which reads out signals in time sequence from the plurality of light receiving elements.
2. Related Background Art
In recent years, in the field of information processing systems, as to one-dimentional image reading apparatus, a development of contact type of equal magnification image sensors in which a plurality of semiconductor optical sensor chips are mounted has been positively pursued in contrast with that of conventional reduction-system line sensors using an optical system.
For example, Japanese Laid-open Patent Application No. 05-227362 discloses a contact-type image sensor which has a newly-added control terminal for resolution control, and the resolution of which can be changed by a user according to a use condition.
FIG. 1 is a circuit diagram of an integrated circuit of the contact-type image sensor disclosed in the above-described Japanese Laid-open Patent Application. In this conventional art, a control terminal 125 is provided on an image sensor chip. To change the resolution of the image sensor chip, a user inputs through the terminal 125 a high-level or low-level signal for selection between a high resolution mode and a low resolution mode.
The circuit shown in FIG. 1 will be outlined below. A start pulse S1 and clock pulses CLK are supplied to shift registers 104. When a shift register 104a is started up by the start pulse S1, an output from the shift register 104a is input to a channel selection switch 103a through a NOR gate 121a and an AND gate 120a. The channel selection switch 103a is thereby turned on to extract a signal from a photocell 101a to a signal line 107a. Other shift registers 104b to 104f are successively started up to output a signal from each of photocells 101b to 101l to the signal line 107a or 107b. 
When a control signal “H” is input to the control signal input terminal 125, analog switches 110a, 110b, 122a, and 122b are switched over to obtain at an image output terminal 111 an image signal of a reading density of 16 dots/mm. When a control signal “L” is input to the control signal input terminal 125, the analog switch 110a is always set in an on state and an image signal of a reading density half the density of the entire photocells 101a to 101l, i.e., 8 dots/mm is obtained at the image output thermal 111. That is, while all the photocells 101a to 101l on the sensor IC are always operating, the image signal can be read out from the photocells with being thinned out a part of the signals according to the control signal. Thus, the voltage level of the image signal is always constant and an image processing circuit of a conventional configuration can be used to process the output image signal read out from the photocells.
In the above-described readout, however, it is necessary to read signals from all the light receiving elements irrespective of the selection between high resolution reading and low resolution reading and to uniformly set a readout time required for reading from all the light receiving elements.